1. Field of the Disclosure
The disclosure relates to semiconductor technology and more particularly to a photomask for fabrication of a thin film transistor (TFT) in a flat panel display (FPD) device.
2. Description of the Related Art
Due to the advantages of a thin profile and low power consumption, liquid crystal displays (LCDs) are widely employed in electronic products, such as portable personal computers, digital cameras, projectors and the like. Typically, such an LCD includes a color filter (CF) substrate, an array substrate, and a liquid crystal layer interposed therebetween, in which each pixel of the array substrate includes TFT circuits.
Typically, photolithography and etching processes are used for the fabrication of the TFT circuits in the array substrate. Different photomasks are needed for each photolithography process. Accordingly, photomasks of 5 to 6 sheets are required for fabrication of the conventional TFT array substrate. Since the photolithography process is expensive and multiple photomasks further increase the manufacturing cost of TFTs, it is desirable to reduce the number of photomasks used for fabricating TFTs.
In order to reduce the number of photomasks used for fabricating TFTs, a proposed approach is to use a single photomask for different etching processes. Namely, a photoresist with different thicknesses may be provided for different etching processes. In the conventional photolithography process, utilization of a halftone mask (HTM) is one of the methods for forming photoresist with different thicknesses. The halftone mask includes a translucent film with different exposure degrees for different regions of a photoresist, such that the photoresist can be formed with different thicknesses.
However, although the number of photomasks used for fabricating TFTs is reduced, process stability using halftone masks is lower than that of binary masks, and thus TFT fabrication yield is reduced. Therefore, there is a need to design a novel halftone mask capable of increasing TFT fabrication yield or maintaining TFT fabrication yield.